In order to increase the magnetic density of high-density magnetic recording medium, it is necessary to reduce the size of the basic unit for recording, but media that utilize conventional sputtered thin films are nearing the upper limits for increasing the recording density due to problems such as thermal decay and increasingly fine crystalline particle sizes and greater dispersion therein. Thus, as candidates for high-density magnetic recording medium, considerable attention has recently been focused on FePt-based magnetic metal nanoparticles that do not have problems with thermal decay and have high anisotropy and exhibit a large coercivity.
Regarding these magnetic metal nanoparticles, Patent Document 1 and Non-Patent Document 1 recite methods of preparing monodispersed FePt metal nanoparticles by performing simultaneous thermal decomposition of iron pentacarbonyl and reduction of platinum(II) acetylacetonate by polyol.
The crystal structure of FePt particles obtained by these methods is a chemically disordered face-centered cubic (fcc) structure, so nano-order particles exhibit superparamagnetism at room temperature. Accordingly, in order for them to be used as ferromagnetic particles, these disordered phases must be subjected to annealing to achieve a crystal structure transition to the chemically ordered L10 phase (face-centered tetragonal (fct) structure).
This annealing requires treatment at a temperature above the phase transition temperature (Tt) from the disordered phase to the ordered phase, and is typically performed at a high temperature above 500° C. In this case, if heat causes coalescence among the particles and an increase in particle size so that the breadth of distribution in the grain size distribution is enlarged, the particles are no longer suitable for use in high-density magnetic recording medium, because of the coexistence of single-domain and multidomain structures. Accordingly, in order to obtain FePt nanoparticles that exhibit ferromagnetism while maintaining the as-prepared grain size of the particles, coating the particles with a protective coating that prevents the coalescence of particles or reducing the Tt by some method so that the annealing can be performed at a lower annealing temperature have been found to be effective.
Non-Patent Document 2 recites a method whereby tetraethylene glycol (TEG) is used as the polyol at the time of preparation of FePt nanoparticles by the polyol method, so when platinum and iron acetylacetonate are reduced at 300° C., FePt nanoparticles with the fct structure are obtained as produced.
Patent Document 1: Japanese Patent No. 3258295 (JP2000-54012A)
Non-Patent Document 1: Science, Vol. 287, 17 Mar. 2000, pp. 1989-1992
Non-Patent Document 2: Japanese Journal of Applied Physics, Vol. 42, No. 4A, 1 Apr. 2003, pp. L350-352
Problems to be Overcome by the Invention
The FePt nanoparticles obtained by the aforesaid method of Patent Document 1 and Non-Patent Document 1 (hereinafter, the method recited in these documents may be referred to as the “IBM method”) have a face-centered cubic (fcc) structure that has no magnetism immediately after the reaction, so they cannot be utilized as magnetic particles as is for magnetic recording medium applications. Thus, it is necessary to subject them to annealing at above the fct phase transition temperature (T1) to achieve a transition to the face-centered tetragonal (fct) structure that exhibits ferromagnetism. At this time, the phase transition temperature for FePt particles obtained by the IBM method is roughly 450° C., so annealing at a temperature above 450° C. is required to cause the transition to the fct structure.
However, if assemblages of these FePt particles (as a powder) are heated as is to a temperature above 450° C., the metal particles will coalesce and result in an increased particle size, so even if the fct structure is obtained, they will not be in a nanoparticle form suited to high-density recording medium applications, and also the coalescence of particles will not typically progress uniformly, thus giving rise to a grain size distribution and an accompanying large distribution in magnetic characteristics, which are practical problems.
In order to prevent the coalescence of particles and increased particle size due to heat, it is necessary to perform this annealing in a state in which the individual particles are positioned so as to have stipulated distances between them, e.g., in a state in which the individual particles are fixed at stipulated positions upon a substrate, or in a state in which there is some sort of barrier that prevents the individual particles from being sintered together. In order to achieve this type of annealing, fine-scale techniques for achieving the regular ordering of particles are required.
In addition, with this IBM method, even in the case of preparing FePt particles having Fe=50 at. % and Pt=50 at. %, for example, the Fe raw material must be charged in a molar amount twice as large, so control of the particle composition is difficult. What should be done to eliminate dispersion in composition among particles is also unknown.
In a FePt alloy, the chemically ordered fct structure of the ferromagnetic phase is limited to the case in which the Pt content is in the range of 35-55 at. %. Accordingly, even if the Pt content is in this range in the average composition of an assemblage of particles, when looking at individual particles, if particles of a composition outside this range are present, then even if those particles are annealed as described previously, they will not have an ordered fct structure. In addition, even if the Pt content of each particle is assumed to be in the range 35-55 at. %, if the composition varies among particles, their magnetic characteristics will also vary, so they will not be suitable for magnetic recording medium.
Furthermore, with the IBM method, even if annealing of particles having the fcc structure is accomplished in the state of being fixed upon a substrate without sintering occurring, it is extremely difficult to orient the easy-magnetization axis of the fct-structure particles thus obtained in a single direction. The reason why is because the individual particles that undergo phase transition to the fct structure upon a substrate are fixed to the substrate and are thus unlikely to move if one attempts to orient their magnetic fields, and also, the temperature at which the fct structure is assumed is higher than the Curie temperature of the FePt particles, so even if annealing is performed in a magnetic field, the easy-magnetization axes cannot be brought into a single direction.
After all, a magnetic substance exhibits the greatest coercivity in the direction of its easy-magnetization axis. When magnetic nanoparticles are disposed upon a substrate, if the easy-magnetization axes are oriented in a single direction, the greatest potential of the magnetic nanoparticles can be achieved and the coercivity of the medium upon which they are disposed can be maximized. Conversely, when the easy-magnetization axes are oriented randomly, the coercivity of that medium is minimized. When looking at a medium with such a random orientation from a certain direction, even if there may be a particle with its easy-magnetization axis pointing in that direction, there will also be a particle with its not easy-magnetization axis pointing in the direction of the minimum coercivity, so the coercivity distribution (SFD value) can be said to be in the most deleterious state. It is well known that decreased coercivity and a deleterious SFD value are disadvantageous to high-density magnetic recording (see Kiroku•Memori Zairyō Handobukku [Recording/Memory Materials Handbook] Tetsuya Ōsaka, Yōtarō Yamazaki, Hiroshi Ishihara, eds., for example). Accordingly, the technique itself of performing annealing after disposing particles with the fcc structure upon a substrate can be said to be incompatible with high-density magnetic recording medium applications. This is the reason why the development of assemblages of independent FePt particles that have the fct structure while also having the freedom of being able to be oriented in a magnetic field is necessary for the development of high-density magnetic recording medium.
Non-Patent Document 2 shows that it is possible to obtain FePt nanoparticles that have the fct structure as prepared. However, powders of FePt nanoparticles obtained by the method recited in this document, even those obtained by the method of preparation using TEG at 300° C., have room-temperature coercivity (Hc) of only 370 Oersted (Oe). In comparison to that prepared at 260° C. using the same tetraethylene glycol (TEG), this FePt nanoparticle powder was confirmed to have the fct structure, but even then its room-temperature coercivity (Hc) is roughly 370 Oe and there is difficulty in its application to actual magnetic recording.
In addition, Non-Patent Document 2 states that FePt nanoparticles having the fct structure as-prepared were obtained, but the individual particles do not necessarily have a uniform composition. In fact, according to Non-Patent Document 2, the metal salts serving as the raw materials for the particles are dissolved in a polyol which serves as both the solvent and reducing agent, the solution is heated up to a stipulated temperature at a constant rate and then held at a stipulated temperature after heating, thereby precipitating the FePt particles. With this method, the crystal nuclei are assumed to be generated continuously over time, so depending on the times when the individual crystals are formed, the ion composition ratio of the metal ions within the reaction solution will be different, and thus it is unavoidable that dispersion will occur in the diameters of the individual particles thus formed and in the sizes of the crystals within the particles, and as a result, it is inevitable that the compositions of the individual particles will differ from each other. Accordingly, even if they may have the fct structure, the individual particles may have a Pt content somewhat greater or less than the alloy composition (average composition) of the entire powder (a distribution appears in the composition of the individual particles and that distribution becomes broad), so some particles may be magnetic and some may not, and even if they are magnetic, their magnetism may be strong or weak.
In the case of magnetic recording, if particles that are not magnetic are mixed in, important data may not be recorded. In addition, ordinary write heads used in magnetic recording are adjusted so as to be able to write to magnetic material having certain magnetic characteristics, so if the magnetism is too strong or weak, phenomena in which information is not recorded well may occur.
Accordingly, an object of the present invention is to solve the aforementioned problems and, in particular, to make further improvements to the method of producing FePt nanoparticles disclosed in Non-Patent Document 2, so as to obtain magnetic material consisting of fct-structure FePt-based nanoparticles with a narrow distribution in composition that are suitable as actual magnetic recording materials.